1. Field of the Invention
The invention relates in general to hoist ring assemblies and, in particular, to a weld mounted hoist ring assembly with a flanged bushing, wherein the perimeter of the flange is welded directly to the surface of an object to be lifted.
2. Description of the Prior Art
Various hoist ring assemblies had been proposed previously. Typically, such hoist ring assembles were designed to threadably engage an object to be lifted. For example, the hoist ring assemblies in Tsui et al U.S. Pat. No. 5,848,815, in Tsui et al U.S. Pat. No. 4,705,422, in Tsui et al U.S. Pat. No. 4,641,986, in Tsui U.S. Pat. No. 5,405,210, and in Wong et al U.S. Pat. No. 4,570,987 all had a protruding mounting screw for threadably engaging an object to be lifted. Generally, to accommodate such prior screw mounted hoist ring assemblies the object must be drilled and tapped to the appropriate thread size and depth before installation. Importantly, it is critical for safety purposes that the depth of the threaded hole is sufficient to provide the proper amount of thread engagement for the hoist ring screw. If the depth is insufficient, catastrophic failure may occur. Thus, hoist ring assemblies with protruding mounting screws are generally unsatisfactory for use in situations where the thickness of the object to be lifted is insufficient to provide enough thread engagement, or, as in watercraft, where holes in the object are undesirable.
Previous expedients proposed for the weld mounting of hoist rings involved a conventional stud welding operation to affix a stud to a metal object. The welded stud replaced the conventional threaded mounting screw. See Sawyer et al. U.S. Pat. No. 5,586,801. The strength of a mounted hoist ring assembly depends in significant part on the mounting structure bearing firmly and uniformly on the load. Any misalignment of a welded mounting stud would prevent this and greatly weaken the mounted assembly. Any weakening of the object by the heat or imperfection of the welding operation is unpredictable so a safety limit for the capacity of the assembly can not be reliably established. The quality of the weld has a great impact of the strength of the system. The weld is concentrated at the end of the stud in a small area, so the loads are likewise concentrated in this small area. The weld mounting of hollow retainer plates for load anchors with limited movement had been proposed. See Smith et al. U.S. Pat. No. 3,831,532.
Previously, difficulties had been anticipated in attempting to weld mount hoist ring assemblies directly to objects to be lifted. Welding had been believed to introduce uncertainty into the resulting load capacity of a hoist ring. For instance, the heat added during welding may destroy the underlying strength of the system. The characteristics of the object have an influence on the strength of the weld. It had been proposed to supply pre-drilled and tapped mounting plates for use with conventional screw mount hoist ring assemblies. These mounting plates were welded directly to the surface of the object to be lifted. The hoist ring assemblies were then threadably mounted to the welded plates. However, due to the wide variety of hoist rings assemblies and their associated lift ratings, a large inventory of various sized, pre-drilled and tapped plates was found to be necessary. Maintaining such an inventory is not only undesirable, but also increases the chances of mismatching an incorrect plate size or capacity for a given hoist ring assembly. Such mismatches are undesirable and can result in catastrophic failure.
Thus, there is a need to provide hoist ring assemblies capable of lifting heavy objects having relatively thin surfaces or surfaces that should not be perforated for attaching the assemblies. There is also a need for such assemblies to be self-contained thereby eliminating the chances of mismatching separately provided threaded plates with conventional screw mount hoist ring assemblies. Those concerned with these problems recognize the need for an improved self-contained hoist ring assembly, one capable of being welded directly to an object to be lifted. The design of the weld must be such that the strength of the resultant weld is reliably and predictably greater than the underlying load rating of the hoist ring assembly so that the weld does not reduce the load rating capacity of the hoist ring assembly.